In order to improve design or decorative property or to impart excellent touch or texture (e.g., soft texture), composites (molded composite articles) formed with a combination of a plurality of resins each having a different hardness, for example, a molded composite article in which at least part of a resin molded article is coated with a thermoplastic elastomer, have been proposed. Such a molded composite article is usually produced by adhesion of a plurality of molded members through an adhesive. For example, Japanese Patent Application Laid-Open No. 267585/1996 (JP-8-267585A) discloses a resin molded article in which a plurality of resin molded articles formed with a polyamide resin or others are adhered to each other through a finishing agent such as a urethane polymer or a urethane adhesive. However, such a process using an adhesive is not only uneconomical due to complicated steps, but also has problems such as environmental pollution by an organic solvent or others.
On the other hand, from the viewpoint of rationalization of production processes or environmental protection, a process for thermal fusing of a plurality of molded members has been adopted. The molded composite article obtained by thermal fusing is usually manufactured by a molding process such as a two-color (or double) molding or an insert molding. However, combination of different materials allowing of thermal fusing is significantly limited. Moreover, it is not easy to establish molding conditions to ensure enough bonded strength. Therefore, in addition to thermal fusing, the fused part is reinforced by a combination use of a process for creating a concavo-convex site (or part) in the composite area of the molded member to join mechanically, or a process for coating a primer or others on the joining (or fusing) part, or other methods. In such a method, however, the molded composite article is deteriorated in flexuous property. For example, the hardened primer layer easily forms a crack by bending. Moreover, the production process needs to complicate the structure of the molded member, resulting in increase of the number of the production steps.
In order to solve these problems, it has been investigated to use a thermoplastic polyurethane as a material for a resin member constituting a molded composite article. The thermoplastic polyurethane itself is relatively excellent in adhesiveness. For example, in a used for shoe(s), a molded composite plastic article comprising a polyamide resin and a thermoplastic polyurethane is practically used as a shoe sole. Moreover, Japanese Patent Application Laid-Open No. 505333/1996 (JP-8-505333A) discloses that a lightened shoe sole is obtained by injection-molding a polyamide elastomer containing a foaming agent into a mold, inserting or putting a molded article of a thermoplastic resin such as a polyether amide, a polyether ester or a polyurethane in a mold, and adhering to the thermoplastic resin molded article (un-lightweight (un-lightened) plastic) and the elastomer (lightweight thermoplastic elastomer). Japanese Patent Application Laid-Open No. 125155/1995 (JP-7-125155A) discloses a molded composite article in which a rigid plastic molded member formed of a blended matter of a polypropylene and a polyamide is coated with a nonrigid (or flexible) plastic containing a thermoplastic polyurethane and a plasticizer by thermal fusing. However, even in such a molded composite article (for example, a molded composite article using a polyurethane resin), the adhesive strength between two materials (e.g., an adhesive strength relative to a polyamide elastomer as a counterpart member) has not been enough yet. Therefore, such a composite is affected by not only conditions for molding or conditions of materials to be used (e.g., production lot) but also environment to be used of the product (molded composite article), resulting in unstableness of the adhesive strength or the duration of the molded composite article (particularly the duration of the adhered site).
It is therefore an object of the present invention to provide a molded composite article in which, even using a polyamide-series resin member and a thermoplastic polyurethane-series resin member different in character from each other, the both members are directly and firmly joined together without an adhesive, and a process for producing the same.
It is another object of the present invention to provide a process for producing a molded composite article in which a polyamide-series resin member and a thermoplastic polyurethane resin member are firmly joined together by thermal fusing in a convenient manner without going through complicated production steps.